Considerable attention has been devoted to the "cavityless technique" of casting. This technique employs a pattern formed from a material that is substantially completely volatile or combustible upon contact with molten metal. Accordingly, the mold body, typically of sand, is arranged with a pouring opening and a vent opening, each communicating with the embedded pattern; upon pouring the molten casting charge into the pouring opening, contact between the entering molten metal with the pattern will cause a rapid volatilization or decomposition of the pattern material so that it is completely destroyed, leaving behind a cavity in a mold body which is in turn filled by the casting charge.
Foamed, thermoplastic resinous materials, such as polystyrene foam, are ideally suited to the production of these fugitive patterns. They possess the necessary strength to remain dimensionally stable during embedment of the pattern in the sand mold body and the actual weight of the material is dramatically small. This, coupled with the volatility of the resinous foam, makes for a rapid and complete burn out of the pattern upon the pouring of the casting charge.
The pattern can be fabricated either from a solid block of such foam resin or by expanding resinous beads to the shape of a pattern die. Cutting blocks to share results in certain rough portions on the pattern surface. If the pattern is molded out of polystyrene beads through a conventional process of steaming and expanding the beads to form a solid mass, the beads tend to define a porous surface where the curved surfaces of separate beads meet. The full explanation for the formation of such porosity is not fully understood, although it is believed that the pressure of steam used to expand the beads, applies a uniform outward force with respect to each bead. But the outer die, in which the beads are expanded offers a continuous resistance surface along all of the beads. The beads are restrained at point contact with respect to each individual bead; the resistance surface does not act uniformly with respect to each bead. As a result, small depressions at the juncture between adjacent beads is produced.
It has been suggested by the prior art that such pattern surface be smoothed by the application of a hot element, such as an iron; this has proven to be of little value since it is impossible to maintain an accurate dimension for the pattern by the pressing technique. It is also been proposed by the prior art to use wax-like coatings which are meltable along with the fugitive pattern upon contact by the molten metal. This also has proven to be of little value because the wax-like coatings are difficult to maintain in an accurately smooth condition prior to casting and volatilization of the wax may occur in advance of volatilization of the pattern thereby retaining the problem as previously encountered.
In the earliest tests to solve the problem of a rough or imperfect surface of a casting formed by a foam pattern, the use of permeable washes were used; they were formed of a slurry of ceramic material consisting typically of pulverulent refractory material, an aqueous dispersing medium and a small amount of binding agent. Such slurries or washes did not prove entirely satisfactory since their purpose was to resist the molten metal and stay solid while the pattern was evaporated. This still does not produce an answer to the problem requiring an ultra-smooth surface since the refractory material, which remained after the pattern was volatilized, possessed the porous-type imprinted surface of the pattern.
Certain critical applications require that castings formed by this technique have an ultra-smooth surface devoid of any defect or undulations. For example, in the making of dynamically loaded elements, such as crankshafts or disc brake calibers, the presence of any slight crevice or pore will promote a site for fatigue fracture to start and will eventually reduce the fatigue life of said element.
Moreover, regardless of how smooth the pattern surface may eventually be made, refractory materials defining the mold cavity will have a particle size which imprints microscopic or small defect sites onto the casting surface since such casting must conform to the sand particle restraint.
Some mechanism must be found to provide an ultra-smooth surface for castings which are made by the cavityless method utilizing foamed fugitive patterns and sand and/or other molding media molds.